Process for producing a display device by deforming thermoplastic spacer particles

ABSTRACT

There is provided a ferroelectric liquid crystal device wherein the yellowing due to a cell thickness increase or occurrence of voids is suppressed. The ferroelectric liquid crystal device includes a pair of substrates each having thereon a group of electrodes for liquid crystal drive, and a layer of ferroelectric liquid crystal disposed between the substrates, wherein thermosetting adhesive particles and thermoplastic polymer particles having a diameter which is 1.5-5 times the liquid crystal layer thickness are dispersed and pressed between the substrates. The polymer particles preferably have a glass transition temperature of at most -20° C.

This application is a division of application Ser. No. 08/150,984, filedNov. 12, 1993 now abandoned, which is a division of application Ser. No.08/008,543, filed Jan. 25, 1993, now U.S. Pat. No. 5,285,304.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to a liquid crystal device using a chiralsmectic liquid crystal, particularly a ferroelectric liquid crystaldevice wherein the increase in cell gap and occurrence of void due tomovement of liquid crystal molecules during multiplexing drive aresuppressed.

A display device of the type which controls transmission of light incombination with a polarizing device by utilizing the refractive indexanisotropy of ferroelectric liquid crystal molecules has been proposedby Clark and Lagerwall (U.S. Pat. No. 4,367,924, etc.). Theferroelectric liquid crystal has generally chiral smectic C phase (SmC*)of H phase (SmH*) of a non-helical structure and, under this state,shows a property of taking either one of a first optically stable stateand a second optically stable state responding to an electrical fieldapplied thereto and maintaining such a state in the absence of anelectrical field, namely bistability, and also has a rapid response tothe change in electrical field. Thus, it is expected to be utilized in ahigh speed and memory type display device and particularly to provide alarge-area, high-resolution display.

For an optical modulating device by use of a liquid crystal having suchbistability to exhibit desirable driving characteristics, it is requiredthat the liquid crystal disposed between a pair of substrates should bein such a molecular alignment state that conversion between the abovetwo stable states may occur effectively irrespective of the applicationof an electrical field.

Further, in a liquid crystal device utilizing birefringence of a liquidcrystal, the transmittance under right angle cross nicols is given bythe following equation (1):

    I/I.sub.0 =sin.sup.2 4θ sin.sup.2 (Δnd/λ)π(1)

wherein

I₀ : incident light intensity,

I: transmitted light intensity,

θ: tilt angle,

Δn: refractive index anisotropy,

d: thickness of the liquid crystal layer,

π: wavelength of the incident light.

The tilt angle θ 0 in the above-mentioned non-helical structure isrecognized as a half of an angle between the average molecular axisdirections of liquid crystal molecules in a twisted alignment in a firstorientation state and a second orientation state. According to the aboveequation, it is shown that a tilt angle θ of 22.5 degrees provides amaximum transmittance and the tilt angle in a non-helical structure forrealizing bistability should desirably be as close as possible to 22.5degrees.

A method for aligning a ferroelectric liquid crystal should desirably besuch that molecular layers each composed of a plurality of molecules ofa smectic liquid crystal are aligned uniaxially along their normals, andit is desirable to accomplish such an alignment state by a rubbingtreatment which requires only a simple production step. As an alignmentmethod for a ferroelectric liquid crystal, particularly a chiral smecticliquid crystal in a non-helical structure, one disclosed in U.S. Pat.No. 4,561,726 has been known for example.

According to our study, it has been found that, in a liquid crystal cellcontaining a ferroelectric liquid crystal, liquid crystal molecules perse are moved in a particular direction within the cell during drive and,as a result, the pressure is increased along a cell side to result in anincrease in cell-thickness. Presumably, such a force causing the liquidcrystal molecule movement may be attributable to an electrodynamiceffect caused by perturbation of liquid crystal molecule dipole momentsin an AC-like electric field caused by continuation of drive pulses.Further, according to our experiments, the directions 22a and 22b of theliquid crystal movement are determined in relation with the rubbingdirection 20 and the average liquid crystal molecular axis position 21aor 21b as shown in FIG. 1A. As the moving direction of liquid crystalmolecules is related with the rubbing direction, the above-mentionedphenomenon is assumed to depend on the pre-tilt state at the substratesurfaces. Referring to FIGS. 1A and 1B, reference numeral 21a (or 21b ina reverse orientation state) denotes an average molecular axis(director) orientation. When the liquid crystal molecules (describedherein as having a negative spontaneous polarization) are oriented toprovide the average molecular axis 21a and are supplied with a certainstrength of AC electric field not causing a switching to the orientationstate 21b, the liquid crystal molecules are liable to move in thedirection of an arrow 22a in the case where the substrates are providedwith rubbing axes extending in parallel and in the same direction 20.This liquid crystal movement phenomenon depends on an alignment state inthe cell as described hereinbelow.

The alignment including a chevron structure of smectic layers may beexplained on a model of two alignment states C1 and C2 shown in FIG. 2.Referring to FIG. 2, reference numeral 31 denotes a smectic layershowing ferroelectricity, 32 denotes a C1 alignment region, and 33denotes a C2 alignment region. A smectic liquid crystal generally has alayer structure and causes a shrinkage of layer pitch when it istransformed from SmA (smectic A) phase into SmC (smectic C) phase orSmC* (chiral smectic C) phase, to result in a structure accompanied witha bending of layers between the upper and lower substrates 14a and 14b(chevron structure) as shown in FIG. 2. The bending of the layers 31 canbe caused in two ways corresponding to the C1 and C2 alignment as shown.As is well known, liquid crystal molecules at the substrate surfaces arealigned to form a certain angle α (pre-tilt) as a result of rubbing in adirection A in such a manner that their heads (leading ends) in therubbing direction are up (or away) from the substrate surfaces 11a and11b. Because of the pre-tilt, the C1 and C2 alignment states are notequivalent to each other with respect to their elastic energy, and atransition between these states can be caused at a certain temperatureor when supplied with a mechanical stress. When the layer structureshown in FIG. 2 is viewed in plan as shown in the upper part of FIG. 2,a boundary 34 of transition from C1 alignment (32) to C2 alignment (33)in the rubbing direction A looks like a zigzag lightning and is called alightning defect, and a boundary 35 of transition from C2 alignment (33)to C1 alignment (32) forms a broad and moderate curve and is called ahairpin defect.

When FLC is disposed between a pair of substrates 14a and 14b and placedin an alignment state satisfying a relationship of H<α+δ. . . (2),wherein α denotes a pretilt angle of the FLC, H denotes a tilt angle (ahalf of cone angle), and δ denotes an angle of inclination of SmC*layer, there are four states each having a chevron structure in the C1alignment state. These four C1 alignment states are different from theknown C1 alignment state. Further, two among the four C1 alignmentstates form bistable states (uniform alignment). Herein, two statesamong the four C1 states giving an apparent tilt angle θ_(a)therebetween in the absence of an electric field satisfying arelationship of H>θ_(a) H/2 . . . (3) are inclusively referred to as auniform state.

In the uniform state, the directors are believed to be not twistedbetween the substrates in view of optical properties thereof. FIG. 3A isa schematic view illustrating director positions between the substratesin the respective states in C1 alignment. More specifically, at 51-54are respectively shown changes in director positions between thesubstrates in the form of projections of directors onto cone bottoms asviewed from each bottom. At 51 and 52 is shown a splay state, and at 53and 54 is shown a director arrangement which is believed to represent auniform alignment state. As is understood from FIG. 3A, at 53 and 54representing a uniform state, the molecule position (director) isdifferent from that in the splay state either at the upper substrate orlower substrate. FIG. 3B shows two states in C2 alignment between whichno switching is observed at the boundaries but an internal switching isobserved. The uniform state in C1 alignment provides a larger tilt angleθ_(a) and thus a higher brightness and a higher contrast than theconventionally used bistable state in C2 alignment.

However, in a ferroelectric liquid crystal having a uniform alignmentstate based on the condition of H>θa>H/2, problem is liable to occur asdescribed below in connection with the above-mentioned liquid crystalmovement.

In an actual liquid crystal cell, the liquid crystal movement occurs asshown in FIG. 1A. For example, when the liquid crystal molecules in theentire cell are placed in a state providing an average molecular axisdirection 21a, the liquid crystal molecules in the cell are liable tomove under AC application in the direction of the arrow 22, i.e., fromthe right to the left in the figure. As a result, the cell thickness ina region 23 is increased gradually to show a yellowish tint. If theliquid crystal molecules are placed in a state providing an averagemolecular axis 21b, the liquid crystal movement under AC application iscaused in the reverse direction 22b. In either case, the liquid crystalmovement is caused in a direction perpendicular to the rubbingdirection, i.e., in the direction of extension of smectic layers.

According to another experiment of ours, when a ferroelectric liquidcrystal cell 60 including a ferroelectric liquid crystal of anon-helical structure in a high pretilt alignment state in a cell regiondefined by a sealant 69 is driven by a driver including a scanningelectrode driver circuit 62 and a data electrode driver circuit 64 tocontinuously write in a white (61) or black (63) fixed pattern as shownin FIG. 4, liquid crystal molecules are caused to move in differentdirections B and C in parallel with the extension direction of chiralsmectic C layers and perpendicular to the rubbing direction Acorresponding to the molecular positions for displaying a white displaystate (region 61) and a black display state (region 63), respectively.As a result, at the destination sides of the liquid crystal molecularmovement, the liquid crystal layer is caused to have an increasedthickness while enlarging the cell gap to form portions 65 colored inpale yellow (yellowing). On the opposite sides of the liquid crystalmolecular movement, voids 67 lacking liquid crystal molecules areformed. These phenomena are caused most noticeably in the neighborhoodof the seal 69 and propagated toward the inside of the cell.

As described above, we have found that the liquid crystal movement inone direction is caused in a long term of drive to cause a slight changein cell thickness, thus resulting in yellowing or color deviationadversely affecting the display characteristics.

More specifically, in the destination side in the liquid crystalmovement direction, the cell thickness is liable to be increased toresult in a yellowish tint. On the opposite side, a state with a lessamount of liquid crystal results to cause a disorder in alignment.Accordingly, a liquid crystal device causing these phenomena as a resultof long term drive shows a uniformity in display performance which ismuch worse than the initial state, thus being accompanied with a problemin reliability.

The above-mentioned change in cell thickness causes further difficultiesincluding changes in drive voltage threshold characteristics and adverseeffects to durability of the liquid crystal device in a long term ofcontinuous drive.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems, an object of the presentinvention is to prevent or effectively suppress the above-mentionedyellowing or occurrence of voids due to a cell thickness increase in aferroelectric liquid crystal device by suppressing the above-mentionedliquid crystal movement.

Another object of the present invention is to provide a ferroelectricliquid crystal device wherein the lowering or deterioration in imagequality or lowering in image display quality due to the above-mentionedyellowing or occurrence of voids can be prevented or effectivelysuppressed.

A further object of the present invention is to provide at a low cost aferroelectric liquid crystal device which is excellent in durability andcapable of displaying high-quality images for a long period.

According to the present invention, there is provided a ferroelectricliquid crystal device, comprising: a pair of substrates each havingthereon a group of electrodes for liquid crystal drive, and a layer offerroelectric liquid crystal disposed between the substrates, whereinthermosetting adhesive particles and thermoplastic polymer particlesrespectively having a diameter which is 1.5-5 times the liquid crystallayer thickness are dispersed and pressed between the substrates.

According to another aspect of the present invention, there is provideda ferroelectric liquid crystal device, comprising: a pair of substrateseach having thereon a group of electrodes for liquid crystal drive, anda layer of ferroelectric liquid crystal disposed between the substrates,wherein thermosetting adhesive particles and thermoplastic polymerparticles having a glass transition temperature of at most -20° C. aredispersed and pressed between the substrates.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are explanatory views for illustrating a cell thicknessincrease along a cell side in relation with liquid crystal movementdirections.

FIG. 2 is an explanatory view for illustrating an alignment stateoccurring in a chiral smectic liquid crystal used in the invention.

FIGS. 3A and 3B are explanatory views for illustrating changes indirector orientation according to various positions between substratesin C1 alignment and C2 alignment, respectively.

FIG. 4 is a front view of a conventional liquid crystal displayapparatus when it is driven for a long time for displaying a fixedpattern.

FIG. 5 is a schematic sectional view of an embodiment of the liquidcrystal cell according to the invention.

FIG. 6 is a graph showing a change in cell thickness increase dependingon the density of dispersed adhesive particles and polymer particles.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the liquid crystal device according to the present invention,thermosetting adhesive polymer particles and thermoplastic non-adhesivepolymer particles are disposed in a pressed state between a pair ofsubstrates sandwiching the liquid crystal material. The thermosettingadhesive particles and thermoplastic polymer particles beforeincorporated in the device or application of an external force theretomay suitably have an average diameter which is 1.5-5 times the cell gap,i.e., the thickness of the liquid crystal layer within the device.

The minimum diameter of the polymer particles of 1.5 times the cell isdetermined because it is a minimum condition for retaining a desiredcolumnar shape between the substrates. On the other hand, the maximumdiameter of 5 times the cell gap has been determined so as to minimizean adverse effect to the alignment from experimental study.

As in a conventional device thermosetting adhesive particles aredispersed to suppress a shearing force to provide an improvedimpact-resistance between a pair of substrates. However, there is acertain limit for the dispersion density of the thermosetting adhesiveparticles due to occurrence of vacuum voids in the liquid crystal layer.In the present invention, the above difficulty is alleviated bydispersion of thermoplastic polymer particles in addition. As thepolymer particles comprise a thermoplastic resin, there is obtained anunexpected benefit that the deterioration of display performances can besuppressed because of an appropriate resilience of the polymer particleseven when the liquid crystal device is exposed to a low temperature.

The polymer particles are pressed and enlarged in area when they aredisposed between the substrates within the liquid crystal device so thatthey effectively suppress the liquid crystal movement.

More specifically, the thermoplastic polymer particles having a diameterof 1.5-5 times the cell, when dispersed and pressed within the liquidcrystal cell containing a ferroelectric liquid crystal, form columnsbetween the substrates, which function as an obstacle to remarkablysuppress the movement of liquid crystal molecules in a ferroelectricliquid crystal layer of, e.g., smectic C phase. If a polymer particlesforms a column of several μm in diameter, this corresponds to severalthousands of layers wherein the liquid crystal molecular movement ishindered in view of the fact that each layer spacing for liquid crystalmovement is several tens of angstromes. When a movement of a liquidcrystal molecule within a layer is suppressed, the molecule cannot butchange moving torque into a direction crossing the layers, but themovement across the layers requires a larger torque by almost one digit.As a result, because of the presence of the obstacle (column), thevelocity of the liquid crystals moving along a layer which is traversedby the column is lowered by nearly one digit. Accordingly, the movementof liquid crystal molecules along the liquid crystal molecular layer issuppressed.

When the adaptability of a ferroelectric liquid crystal panel to storageat a lower temperature is considered, in case where rigid columnarstructures are present within the panel, the columnar structures under avolumetric shrinkage between the substrates upon cooling and on theother hand the liquid crystal contained therein causes a shrinkageregardless of the columnar structures. As a result, there can be formedvoids within the cell in some cases because the cell shrinkage fails tofollow the liquid crystal shrinkage.

In order to cope with the problem, in the present invention, thecolumnar structures may preferably be provided with a resilience orsoftness by using non-adhesive thermoplastic polymer particles inaddition to the adhesive particles used for providing an increasedimpact resistance. In order to provide a better low-temperaturestorability, the polymer particles may preferably comprise athermoplastic polymer having a glass transition temperature Tg of atmost -20° C. On the other hand, the polymer particles are required to bedispersed within the substrates and accordingly required to have anappropriate dispersibility. The low glass transition temperaturecharacteristic is generally contradictory with the dispersibility. Thecontradiction can be removed by micro-encapsulating polymer particleshaving such a low glass transition temperature.

Hereinbelow, some embodiments of the present invention will be describedwith reference to drawings.

FIG. 5 is a partial schematic sectional view of a liquid crystal cellaccording to a suitable embodiment of the present invention. Referringto FIG. 5, the liquid crystal cell includes a pair of oppositelydisposed glass substrates 5 respectively having on their inside surfacesITO stripe electrodes 3 extending in mutually crossing directions, metalelectrodes or wires 4 disposed on the ITO electrodes 3, insulatinglayers 2 and alignment films 1 further disposed inside, polymerparticles 6 dispersed and pressed in adhesion between the alignmentfilms 1, and a ferroelectric smectic liquid crystal 7 placed in anon-helical structure providing at least two stable states and fillingthe gap between the alignment films 1. In FIG. 5, the thermosettingadhesive particles are omitted from showing.

The polymer particles 6 used in the present invention may suitablycomprise a resilient material not reactive with the liquid crystalmaterial used. Examples of such material may include: resins, such ashigh-density or low-density polyethylene, acrylic urethane, nylon,acrylic resin containing a rubber, such as butyl rubber, added thereto,and silicone rubber. These materials are not exhaustive and can also beused in combination of plural species.

The thermosetting adhesive particles may preferably be dispersed at arate of at least 40 particles/mm² more preferably at least 80particles/mm² in order to provide a required impact resistance andgenerally at most 160 particles/mm² so as to avoid occurrence of vacuumvoids. On the other hand, the thermoplastic non-adhesive polymerparticles may preferably be dispersed at a rate of at least 40particles, more preferably at least 80 particles/mm² and at most 140particles/mm² so as hot to cause deterioration in image quality.Accordingly, the polymer particles may preferably be dispersed at least80 particles/mm² more preferably at least 160 particles/mm² and at most300 particles/mm² in total.

EXAMPLE 1

In order to form a columnar structure of polymer particles 6, nylonresin particles ("SP-500M", mfd. by Toray K. K.) having an averagediameter of 5 μm which was about 4 times a pre-set cell gap of 1.2 μmwere suspended in IPA (isopropyl alcohol) solution and, after sufficientdispersion, the resultant dispersion was sprayed with nitrogen gasthrough a spray nozzle to be dispersed on an alignment film 1 of asubstrate. Further, thermosetting adhesive particles of a similardiameter were dispersed at a rate of 80 particles/mm² as usual. Then,through an ordinary cell preparation process including application ofanother substrate with a sealant and curing of the sealant, a liquidcrystal cell having a structure as shown in FIG. 5 was prepared. In thesealant curing step, the polymer particles were melted and pressedbetween the substrates to form a columnar structure as illustrated.

EXAMPLE 2

Several cells were prepared in the same manner as in Example 1 exceptthat the dispersion rate of the thermosetting adhesive particles wasfixed at 80 particles/mm² and the dispersion rate of the polymerparticles were changed in various ways.

The resultant cells were subjected to 100 hours of drive test underapplication of rectangular pulses of Vop=20 volts at 30 ° C., and theresultant maximum cell increase for each cell was measured. The resultsare shown in FIG. 6.

As is shown in FIG. 6, the cell thickness change increase Δd wasdecreased up to a total dispersion density of 200 particles/mm² and at ahigher dispersion density the cell thickness change was notsubstantially decreased any further.

EXAMPLE 3

Encapsulated polymer particles were prepared by micro-encapsulatingparticles of acrylic methane resin having an average diameter of 5 μmwhich was about 4 times a set cell gap of 1.2 μm with PMMA (polymethylmethacrylate). Then, the encapsulated polymer particles were suspendedand well dispersed in IPA solution and the resultant dispersion wassprayed with nitrogen gas through a spray nozzle to be dispersed at arate of 80-140 particles/mm² on an alignment film 1 of a substrate.Further, thermosetting adhesive particles were dispersed at a rate of 80particles/mm² as usual.

Then, through an ordinary cell application process, a liquid crystalcell (panel) having a sectional structure as shown in FIG. 5 wasprepared. During the cell preparation, the encapsulated polymerparticles were pressed between the substrates to rupture the capsulesand exposed the soft polymer particles therein in the step of sealantcuring under pressure.

Then, the liquid crystal panel thus prepared was stored at a lowtemperature of -20 ° C. As a result, occurrence of voids within thepanel was not observed even after lapse of 240 hours. In contrastthereto, a liquid crystal panel prepared in a similar manner except forusing polymer particles having a higher glass transition temperature(i.e., in the same manner as in Example 1) caused voids therein in 12hours.

The above results show that the use of encapsulated polymer particleshaving a low glass transition point for dispersion within a cell couldprevent occurrence of voids during low-temperature storage whilepreventing the occurrence of yellowing or voids during operation.

EXAMPLE 4

Several cells were prepared in the same manner as in Example 3 exceptthat the dispersion rate of the thermosetting adhesive particles werefixed at 80 particles/mm² and the dispersion rate of the polymerparticles were changed in various ways.

The resultant cells were subjected to 100 hours of drive test underapplication of rectangular pulses of Vop=20 volts at 30° C., and theresultant maximum cell thickness increase for each cell was measured inthe same manner as in Example 2.

Similarly as in Example 2, the cell thickness change Δd was decreased upto a total dispersion density of 200 particles/mm² and at a highdispersion density the cell thickness change was not substantiallydecreased any further.

EXAMPLE 5

Three liquid crystal cells were prepared in the same manner as inExample 3 except that the polymer particles of acrylic resin containing35% of butyl rubber, silicone rubber resin (mfd. by Toray Silicone K.K.) and low-density polyethylene resin (mfd. by Sumitomo Seika K. K.),respectively, were used.

Then, the liquid crystal cells were subjected to a similarlow-temperature storage test at -240° C. as in Example 3. In any case,no abnormality was observed even after 240 hours of the storage. Incontrast thereto, a liquid crystal cell prepared in the same mannerexcept for the use of polymer particles having a glass transitiontemperature exceeding -20° C. caused voids after 12-24 hours of thestorage.

As described above, according to the present invention, it is possibleto remarkably suppress the occurrence of yellowing or voids due tomovement of liquid crystal molecules even after a long time of drive bydispersing thermoplastic polymer particles having a diameter 1.5-5 timesthe liquid crystal layer thickness between the substrates in a pressedstate in addition to thermosetting adhesive particles.

Further, by using thermoplastic polymer particles having a glasstransition temperature of at most -20° C., it is possible to provide animproved low-temperature storability while retaining the effect ofsuppressing the yellowing or occurrence of voids.

Further, by using such polymer particles in an encapsulated form, it ispossible to improve the dispersibility of the polymer particles wherebyit is possible to enhance the effect of suppressing liquid crystalmovement and further improve the display quality.

What is claimed is:
 1. A process for producing a liquid crystal devicecomprising, in this order, the steps of:dispersing resilientthermoplastic polymer particles at a density of at least 80particles/mm² on at least one of a pair of substrates, said polymerparticles having a diameter (d) and a glass transition point of at most-20° C., superposing the pair of substrates opposite to each other so asto sandwich the thermoplastic polymer particles under application ofheat and pressure to deform the thermoplastic polymer particles into acolumnar shape and attain a prescribed gap (g) between the substrates,wherein said prescribed gap (g) and said particle diameter (d) satisfythe relationship d/5.0<g<d/1.5; and filling the gap between thesubstrates with a chiral smectic liquid crystal.